The neural voices available in Storyline are quite realistic and can be very helpful to test timings and offer example content to be signed off by a client. You may then add real voice narration to create a more professional final product.
Automatic audio narration can be annoying for users who would prefer it not to automatically play. You could create a button to allow the audience to only play narration when they want to:
1: Add a new button to a slide and label it appropriately:

2: Create a new TRIGGER TO PLAY MEDIA, and select the TEXT TO SPEECH track:

From here you could create a single ‘Would you like narration?’ button on one of your first slides then update a variable to ‘remember’ if the user would like narration or not. See section 8 for examples of variables and where to find more information.